Recycle bioreactor for bioethanol production from wheat starch II. Fermentation and economics

Bioethanol has been produced using sugars from cold hydrolysis of pure wheat starch, sterile wheat flour, and unsterile wheat flour. The conversion of sugars to ethanol from pure starch reached 96% of the theoretical maximum while that from sterile wheat flour was 69% and from unsterile wheat flour only 35%. These data indicate that sequential hydrolysis and fermentation of wheat flour is not feasible. However, the simultaneous cold hydrolysis and fermentation of both wheat starch and wheat flour proved successful. Both sugar sources produced 95% of the theoretical maximum amount of ethanol. The process could be repeated in a sequential batch fashion for over 110 h of operation, achieving high ethanol yields in each run. A scale-up design of bioethanol production using sequential bath, simultaneous cold starch hydrolysis and fermentation indicates that this process would be economically feasible. High levels of revenue are generated from both the bioethanol and the byproduct: food-grade wheat gluten. The payback period is predicted to be <2 years with a discounted cash flow rate of return of 46%.